This invention relates generally to new and improved means for propelling a vehicle, and it relates more particularly to a propulsion method and system for an internally powered traction vehicle that is driven by the combination of a prime mover, electric power generating means, and adjustable speed a-c electric motors.
In a large self-propelled electrically driven traction vehicle, such as a locomotive or an off-highway truck, the wheels of the vehicle are propelled (or retarded) by electric motors energized by electric power generated by a rotating electrodynamic machine that in turn is driven by an on-board thermal prime mover such as a diesel engine. See for example U.S. Pat. No. 3,878,400--McSparran. Heretofore traction motors commonly have been of the direct current (d-c) variety, and the necessary d-c excitation for such motors has been supplied either by using a d-c generator or, alternatively, by rectifying the output of an alternating current (a-c) generator.
Because a-c motors are generally more simple and compact than d-c motors, are not limited by commutator constraints, are capable of greater tractive effort than d-c motors at high speeds, and are relatively light weight, low cost, and easy to maintain, persons working in this art have been giving increasing attention to utilizing adjustable speed a-c traction motors rather than d-c motors in drive systems for electrically propelled traction vehicles. In an a-c propulsion system, traction motor speed (and hence vehicle speed) is dependent, in large measure, on the fundamental frequency of the excitation supplied to the stator windings of the motors, and in order to control the frequency it has been proposed to supply excitation to the motors via variable-frequency electric power static inverters or frequency changers formed by a plurality of controllable electric valves or semiconductor switching elements (e.g., thyristors) of the kind having the ability to hold off forward voltage until turned "on" in response to a suitable firing or gate signal. Once a valve is triggered or fired by its gate signal, it switches from a blocking or non-conducting state to a forward conducting state in which it can freely conduct load current until this current is subsequently transferred or commutated to a companion valve in the power conversion apparatus. In the case of an inverter, the valves are arranged in alternative paths of loadcurrent conduction between a set of a-c terminals (which are connected to the stator windings of the a-c motor) and a pair of d-c terminals (which are adapted to be coupled to a suitable d-c power supply), and they are cyclically fired in a predetermind sequence so as to convert d-c power into a-c power having a fundamental frequency determined by the switching frequency of the valves. Either voltage or current source inverters can be utilized
With a voltage source inverter, the amplitude and frequency of the fundamental alternating voltage that is supplied to the stator terminals of the associated a-c motors are controlled, and stator current can vary widely in magnitude. With a current source inverter (hereinafter also referred to as a controlled current inverter) the quantities that are controlled are the amplitude and frequency of alternating current exciting the stator windings, and the voltage magnitude can change rapidly during commutation. A controlled current type of inverter is required in practicing the present invention. For a particular example of a circuit well suited for this purpose, see the improved auto-sequential commutated inverter that is described and claimed in U.S. Pat. No. 3,980,941--Griebel.
As is well known to persons skilled in the art, there are fundamental differences between current and voltge source inverters. (See. e.g., "Survey of Controlled Electronic Power Converters" by W. McMurray, pp. 39-62 of survey papers of IFAC Symposium, Duesseldorf, Germany, October 1974.) For the convenience of the reader, the distinguishing characteristics of these two classes of inverters will now be reviewed.
In the case of a voltage source inverter (sometimes also referred to as a "voltage fed" or "controlled voltage" inverter), the d-c terminals of the inverter need to be coupled to a source of direct voltage characterized by relatively constant magnitude and unchanging polarity. In practice, the voltage fed inverter is supplied from a fixed or adjustable source of relatively smooth direct voltage without an intervening current-smoothing choke of significant size, and the inverter valves are cyclically fired in a predetermined sequence so that square wave or quasi-square wave voltages are applied to the load. (For this discussion the load is assumed to be a polyphase a-c traction motor.) The magnitude of motor current will depend on both the applied voltage and the motor impedance, and it can vary widely.
Because of the reactive nature of the motor load, motor current is not in phase with motor voltage and there are cyclic periods when the direction of current reverses with respect to the polarity of the applied voltage. Therefore, in a voltage fed inverter, feedback rectifiers (diodes) are connected across the main controllable valves of the inverter to accommodate the reverse current flow, and the input current to the inverter will have an a-c component. The d-c link that interconnects the direct voltage source and the d-c input terminals of the voltage fed inverter must include a "sink," which is ordinarily a parallel capacitor, for alternately accepting and delivering this a-c component of current. The parallel capacitor also serves to smooth the input voltage where the output of the direct voltage source is in the form of rectified alternating voltage having a high ripple content. To assist this voltage smoothing or filtering action of the parallel capacitor, the d-c link will usually include a small series inductor which may also be necessary to prevent capacitor inrush current from exceeding a level that would damage the valves of the rectifier circuit. Since the parallel capacitor maintains the voltage on the d-c terminals of the inverter substantially equal to the average magnitude of voltage at the output of the voltage source, the maximum voltage absorbed by the series inductor in the d-c link is relatively low. Therefore the total series inductance in the d-c link of a voltage fed a-c motor drive system can be relatively small, and in fact it must be small to avoid impeding the required reversal of current in the link in the event of regeneration when the traction vehicle is braking and the motor is operating as a generator driven by mechanical inertia.
In summary, the functions of the capacitor on the d-c link of a voltage fed system (i.e., a system utilizing a voltage source inverter) are to provide a low impedance path for the a-c component of link current and to limit the rate of change of direct voltage that is applied to the d-c terminals of the inverter. More specifically, the capacitance provided by the filter in the d-c link, taken together with any other inherent or stray capacitance of the link, reduces the steady-state ripple content of d-c link voltage. As a general rule, the filter is sized to limit ripple voltage to less than approximately 15 percent of the average magnitude of voltage on the d-c link of the voltage fed induction motor drive system, and in the case of electrolytic capacitors even greater voltage smoothing action is obtained because the filter has to be large enough to conduct, without overheating, the peak a-c component of current in the d-c link.
In contrast to a voltage source inverter, a controlled current inverter must have its d-c terminals coupled to a source of relatively smooth direct current, and this current is restricted to one direction in the interconnecting d-c link. Reversal of power flow during electrical braking is accomplished by reversing the polarity of the direct voltage. As the respective valves of the controlled current inverter are fired in sequence, link current is commutated from phase to phase in the stator windings of the traction motor connected to the a-c terminals of the inverter. Consequently the inverter supplies the motor with square wave or quasi-square wave currents. The amplitude of motor current is determined by the magnitude of current supplied by the direct current source, while motor voltage is determined by motor impedance and counter electromotive force and can vary widely. Therefore in a current source inverter the voltage at the d-c input terminals of the inverter has to be unconstrained and free to vary at will, and the inverter would be subject to material change in its behavior if appreciable capacitance were provided in parallel with its d-c terminals for smoothing this voltage.
The current source of a controlled current inverter is suitably controlled or regulated so as to set and maintain a desired magnitude of direct current. In an internally powered vehicle having an on-board prime mover, the d-c source would logically comprise a rotating d-c generator having its armature connected to the d-c terminals of each controlled current inverter by way of a d-c link including a current smoothing filter, and current magnitude would be regulated by appropriately controlling the electromagnetic excitation of the generator to thereby adjust the magnitude of voltage that the generator impresses on the d-c link. The current smoothing filter ordinarily is in the form of a series inductor (also referred to as a reactor or choke), and its functions are to absorb the short duration, high magnitude voltage transients that periodically occur between the source and the respective d-c terminals of the controlled current inverters and to limit the rate of change of direct current that is supplied to each separate inverter-motor set. More specifically, the inductance provided by the filter in the d-c link, taken together with any other source inductance and with the motor inductance, reduces the steady-state ripple content of motor current so as to minimize certain resulting torque harmonics in the traction motor. As a general rule, the filter is sized to limit ripple current to approximately 10 to 20 percent of the average magnitude of current in the d-c link of the current fed induction motor drive system.
Insofar as I am presently aware, persons skilled in the art have not previously recognized that in a traction vehicle having a self-contained current source a-c motor drive, the current smoothing filter can be omitted altogether from the d-c link, thereby realizing a significant reduction in size, weight, and cost of the propulsion system, if the d-c generator were replaced by the rectified output of an a-c generator of the kind heretofore used for traction vehicle applications.* To better understand this discovery, the characteristics of an a-c generator that I believe make it uniquely well suited to be used in combination with a controlled current inverter will now be briefly reviewed. FNT *The rectified output of a diesel engine-driven 3-phase alternator has heretofore been proposed as the d-c power supply for voltage source inverters in an a-c traction motor propulsion system for locomotives. See the paper entitled "A New Converter Drive System for a Diesel-Electric Locomotive with Asynchronous Traction Motors" by J. Brenneisen et al, published in IEEE Transactions on Industry Applications, Vol. IA-9, No. 4, pp. 482-90 (July/August 1973). The voltage source inverters of the Brenneisen et al system are typified by a voltage smoothing filter (capacitor C.sub.d in FIG. 6) connected in parallel with their d-c terminals and by the pulsewidth modulated voltage control shown in FIGS. 13 and 14 and described on pages 485 and 488 of the paper. As was explained above in the review of voltage source inverters, series inductors of relatively small inductance values are usually provided in the d-c links of such systems to enhance the voltage smoothing action of the parallel capacitor. Brenneisen et al suggest (page 485) that the reactances of the alternator will act in place of such prior art chokes. Not that the present invention relates to a current source a-c motor drive differing significantly in general character, and especially operationally, from voltage source systems such as disclosed by Brenneisen et al, and there is nothing in the referenced paper to indicate any awareness by the authors that a current source system could be developed to operate successfully without the large inductor heretofore conventionally provided in the d-c link to smooth the current delivered to controlled current inverters.
An a-c generator, of the type known as a synchronous generator, is a machine having an armature winding in which alternating current flows and a field winding to which d-c excitation is supplied. The armature winding usually is on the stator, and in a 3-phase machine the windings of the individual phases are displaced from each other by 120 electrical degrees in space around the circumference of the stator-rotor air gap. The field winding is located on the rotor, which can be of either salient-pole or cylindrical construction, and the field poles are excited by direct current brought in through slip rings or by a brushless exciter. The field produced by the d-c rotor winding revolves with the rotor. If the rotor is driven by a prime mover to which it can be mechanically coupled, the magnetic field produced by the rotor winding will induce in the stator windings an alternating voltage having a frequency proportional to the number of poles and the angular velocity of the rotor. Ordinarily the stator windings are distributed in such a manner that the alternating voltage has a generally sinusoidal wave form, but they can alternatively be arranged to generate other wave forms if desired.
Synchronous generators designed specifically for traction vehicle applications (hereinafter referred to as traction alternators) ordinarily are high-reactance machines. By high reactance I mean that the armature reaction (in ampere-turns per pole) of the machine at full-load current is a large percentage (e.g., 200%) of the no-load field ampere-turns at rated voltage and frequency. Armature reaction refers to the effect of magnetomotive force (mmf) resulting from current in the armature windings. The armature mmf modifies the electromagnetic flux produced by current in the field windings and changes the strength of the resultant field in the stator-rotor air gap of the machine. If an electric load connected to the armature terminals of a traction alternator were short circuited and the field excitation were held constant, the armature mmf will almost directly oppose the field mmf, thereby demagnetizing or weakening the resultant air gap field and limiting the rise in armature current. In effect the rate of change of current is limited by a transient reactance similar to the equivalent reactance of a short-circuited transformer. The final value of short circuit current is proportional to the synchronous impedance of the machine.
A typical traction alternator at normal excitation and speed has sufficiently large reactance to limit steady-state short circuit current to less than rated full-load current. This reactance is sometimes referred to as the steady-state unsaturated synchronous reactance x.sub.s of the alternator. In practice its per unit value is in a range from 1.0 to 3.0, and therefore a typical traction alternator will tend to maintain rated current with changing load impedance.
Representative of traction alternators suitable for the practice of my present invention is General Electric model GTA22 manufactured by the General Electric Company, Transportation Systems Business Division in Erie, Pa. The horsepower rating of this machine is approximately 1,200 at rated engine speed. Contributing to the relatively high synchronous reactance of the GTA22 alternator is the fact that it has no amortisseur windings. Amortisseur windings are short-circuited damper bars or squirrel-cage windings that are often inserted in the rotor pole faces of synchronous generators for the purpose of producing torques that help to damp out mechanical oscillations of the rotor about its equilibrium position. Such windings also improve the transient voltage regulation of the machine. By voltage regulation I mean the tendency of the voltage amplitude at the terminals of the stator windings to remain substantially constant, under conditions of constant excitation and frequency, regardless of variations in the electrical load connected across the terminals. When amortisseur windings are used in a synchronous generator, they effectively oppose changes of electromagnetic flux in the air gap and thereby reduce the rate at which terminal voltage transiently varies in response to rapid changes in the load. In some applications (e.g., electric utilities where a-c generators are operated in parallel with one another to supply interconnected power distribution systems), the damping and good voltage regulation provided by amortisseur windings are desired, but in traction applications these features are not needed, and thus traction alternators ordinarily do not require the use of amortisseur windings. As a result of its high synchronous and its omission of amortisseur windings, a traction alternator is specially well suited for use in combination with controlled current inverters supplying adjustable speed a-c traction motors.